The crest factor of a modulated signal is generally defined as the ratio of its peak power to its average power. Constant envelope modulation techniques such as frequency modulation (FM) and Gaussian minimum shift keying (GMSK) have a low crest factor. Therefore, in conventional communication systems, these techniques have been employed to allow a transmitter power amplifier to operate in its non-linear range near saturation, resulting in high power efficiency. However, these modulation techniques exhibit low spectral efficiency.
The introduction of third generation (3G) code division multiple access (CDMA) systems such as wideband CDMA (WCDMA) and CDMA2000 has driven the need for high spectral efficiency modulation schemes such as phase shift keying (PSK) and quadrature amplitude modulation (QAM), capable of handling the increased volume of mobile users. In 3G base stations, after PSK or QAM modulation, signals associated with multiple carriers are combined into a single wideband signal which is then amplified by a multi-carrier power amplifier. Due to the high crest factor caused by the use of spectrally efficient modulation techniques and multi-carrier combining, the wideband signals are subject to severe in-band and out-of-band distortion when the multi-carrier power amplifiers are operated outside their linear range. Typically, to avoid such nonlinearity, the output power of the multi-carrier power amplifier must be reduced or “backed off,” resulting in very low power efficiency.
It is well known that the efficiency of the multi-carrier power amplifiers is an important determinant in the cost of 3G base stations. Thus, when the crest factor increases, the cost of the base stations also increases. Cost is increased because the low power efficiency of the power amplifier may require higher power consumption in order to achieve a desired coverage level in the system. Accordingly, crest factor reduction (CFR) has become a very important task in the design of 3G base stations.
Crest factor reduction is generally a baseband signal processing technique, which aims to reduce the dynamic range of a signal prior to power amplification. It is usually combined with another power amplifier linearization technique, such as digital pre-distortion (DPD), in order to achieve the best power amplifier efficiency.
A number of crest factor reduction techniques are known in the art. These include peak clipping and peak windowing. As indicated above, these techniques are applied to baseband signals prior to power amplification. For example, such techniques may be applied in a baseband signal processor of a base station transmitter in a WCDMA or CDMA2000 cellular system.
Conventional peak clipping of an input signal x(n) to generate a clipped signal y(n) can be expressed as the following multiplication:
                    y        ⁡                  (          n          )                    =                        c          ⁡                      (            n            )                          ⁢                  x          ⁡                      (            n            )                                ,                  ⁢    where              c      ⁡              (        n        )              =          {                                                  1                                                                          A                                                                        x                    ⁡                                          (                      n                      )                                                                                                                            ⁢                                                                                                                    x                    ⁡                                          (                      n                      )                                                                                        ≤                A                                                                                                                                                x                    ⁡                                          (                      n                      )                                                                                        >                A                                                        ⁢                          denotes the clipping factor for the nth sample x(n) of the input signal, and A is the maximum amplitude of the clipped signal y(n). The clipping ratio is given by A/σ, where σ is the standard variance of the input signal x(n) before clipping. Unfortunately, conventional clipping techniques can cause sharp corners in the clipped signal and thereby significantly degrade the adjacent channel leakage ratio (ACLR). Typically, a complex long-tap filter with sharp transition characteristics is required for filtering subsequent to the clipping process. Furthermore, peak re-growth after filtering typically limits the reduction in crest factor that is achievable with a peak clipping approach.
In general, the peak windowing approach involves multiplying the signal to be clipped, x(n) in the example above, with a windowing function. This allows the sharp corners to be smoothed, improving the ACLR. However, conventional peak windowing is disadvantageous in that it can produce a high error vector magnitude (EVM), which is primarily caused by over-clipping the signal in the presence of frequent peaks. The 3G WCDMA specifications call for an upper bound of 17.5% EVM, which limits the effectiveness of conventional peak windowing in terms of its ability to reduce crest factor.
Accordingly, what is needed is an improved peak windowing approach which can provide a substantial reduction in crest factor without unduly increasing EVM.